RNA-directed DNA polymerase isolated from avian myeloblastosis virus has been used successfully to synthesize full size complementary DNA to rabbit globin mRNA and chicken procollagen mRNA. These specific DNA molecular probes are subsequently recombined to a suitable vector for amplification by molecular cloning. The probes are then used to identify large genomic fragments of DNA which contain the gene sequences. It is the purpose of this project to elucidate the differences in the DNA sequences between normal and beta-thalassemic tissues. Hopefully, through the future application of these methodologies, a fuller understanding of the broad spectrum of human genetic disease will result.